This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The present invention relates to a mobile, year-round arctic drilling system, also referred to herein by the acronym “MYADS.” It is a drilling system for drilling offshore wells and/or performing other offshore activities at multiple, successive locations in a “sub-Arctic” environment. The system combines the ability to move to different locations and the strength to resist ice loading when on location and when ice-covering is present in the sub-Arctic environment.
The “sub-arctic” offshore environment is characterized by yearly, seasonal incursions of ice. This environment is less severe than that of the “high” arctic environment that may have ice present year-round. However, even the sub-arctic environment presents problems for the use of standard offshore drilling systems. The standard offshore drilling systems are primarily designed to resist loading from waves, winds and currents, and, where necessary, earthquakes, but not from ice. In a sub-arctic environment, the overall or global loading due to ice impingement on an offshore drilling system could be an order of magnitude higher than that associated with wave, wind and current loading. Thus, the structure of a typical offshore drilling structure would not able to withstand the significantly higher forces in a sub-arctic environment.
Ice impingement can also create large pressure forces in small, local areas of any drilling equipment structure. For a typical offshore drilling system, these high local forces would damage unprotected frame brace elements since these elements are typical offshore structures designed solely to resist wind, waves and current.
The advantage of mobility is that it allows the drilling equipment to operate at widely different locations without the need to build a permanent structure to support the drilling equipment at each location.
Some current drilling structures have been designed for sub-arctic conditions. However, most of these structures are configured as permanent (non-mobile), production/drilling/quarters (PDQ) platforms. Various kinds of icecrush resistant drilling structures are also known. Brick-type systems, such as the Concrete Island Drilling System (CIDS) described in U.S. Pat. No. 4,011,826, are one type of an ice crush resistant structure. Another example is the structure disclosed in U.S. Pat. No. 5,292,207. Each of these systems is a large, permanent, walled structure configured to receive drilling rigs.
Other existing systems require some major structural components to be permanently on location (i.e., only the drilling facilities themselves are mobile). One example is the Deck Installation System for Offshore Structures disclosed in U.S. Pat. No. 6,374,764. Another example is the monopod jack-up configuration disclosed in U.S. Pat. No. 4,451,174. In these systems, a different sub-structure anchored to the seabed is required for each new drill location.
Another example of a monopod jack-up system is the offshore platform erection system and method of U.S. Pat. No. 4,648,751, which utilizes a single leg attached to a permanently installed substructure. The single-leg structure is jacked up by a retractable jacking system. Once at operating height, the deck is secured to the single leg, and the drilling derrick is moved into position to drill. The monopod jack-up is intended to drill exploration wells in an arctic environment. However, this configuration is only designed for exploration drilling with no provision for re-deployment over an active well site. Further, the single-column design may not be structurally sound for seismically-active locations.
Existing mobile drilling systems for non-arctic conditions, such as the conventional jack-up system, cannot operate in areas where the structure may come into contact with ice floes. There are two types of such conventional jack-ups: (1) those supported on open lattice structural legs and (2) those supported on closed cylindrical legs. Neither of these existing designs is capable of resisting local and global loading due to sub-arctic ice.
The open-lattice leg design is not suitable to resist the local ice forces as individual members of the lattice structure would be bent or crushed by the local ice forces. The closed-cylindrical leg design improves on this drawback. However, current designs are not suitable to resist the high local ice loads as the legs are primarily designed to resist much smaller wave loading. Some current closed-cylindrical leg designs have moments of inertia as low as 1.1 meters to the fourth power (m4).
Neither of the above designs is capable of resisting the global ice loads typical of sub-arctic regions. These global ice loads can easily be an order of magnitude higher than the wave and wind loads to which conventional jack-ups are designed to resist.
Accordingly, a need exists to configure a structure that can support offshore drilling operations while able to withstand both global and local ice loading that will occur during the yearly, seasonal incursions of ice. In addition, the structure should have the capability to relocate to a new drilling site during the relatively ice-free time of the year, and return, if necessary. Preferably, the relocation time may be relatively short and require no significant offshore logistics support (i.e., nothing more than a few towing vessels).
Other related material may be found in at least U.S. Pat. No. 4,249,619; U.S. Pat. No. 5,228,806; U.S. Pat. No. 5,288,174; and U.S. Pat. No. 5,290,128.